"We saw some indication of noise for a period of about five minutes that we had not seen before," said Richard Stickler, director of the U.S. Mine Safety and Health Administration. "We're not sure what it means."

Bob Murray, president and CEO of mine co-owner Murray Energy Group, told reporters, "Don't read too much into this noise we picked up."

"But it is a sign of hope," he said.

Stickler said that based on the location of the unknown noise, rescuers have decided to change the location of a fourth drill hole they are planning to bore down into the mine in an effort to make contact with the miners and determine the conditions inside. The new drilling is expected to begin Thursday morning. See photos of the rescue efforts »

Wednesday evening, a video camera and microphone were lowered into a third bore hole, which was completed earlier in the day. Mine officials told CNN that video from the scene showed a brattice curtain, which miners can use as a short-term refuge for fresh air in the event of a cave-in. It was unclear if the curtain was newly erected or had been set up at an earlier time.

Workers planned to insert a steel pipe into the hole to draw air out of the mine for testing after the camera and microphone were pulled out.

"That will tell us whether there is an atmosphere that will sustain human life," Murray said.

During the drilling of the third bore hole Wednesday morning, two of the six "geophone" listening devices on the mountain above the mine detected the unknown noise, Stickler said. It lasted for about five minutes, stopped and has not been detected since, he said.

Geophones have never been used successfully to rescue trapped miners, but, in experiments, they have been able to pick up signals as far as 2,000 feet under the surface, Stickler said.

There has been no contact with the six miners since they were trapped August 6.

Even if the trapped miners are found to be alive, it will be days before they could be brought out. At least 1,100 feet of tunnel in a 2,000-foot stretch -- the only way out -- remain to be cleared through many tons of coal rubble.

The work is made tedious by the need for a team of miners to follow the machine, shoring up the tunnel walls with high-pressure jacks, chain-link fencing and wooden beams along the way.

It's expected to take at least another week -- barring more problems -- before the tunnel is cleared enough to allow a search of the area, rescuers have said.

However, that work has been slow going, hampered by unstable conditions and seismic shifts inside the mine.

While Murray continued to insist an earthquake caused the mine collapse and it was not related to mining activity, seismologists at the U.S. Geological Survey have said no earthquake occurred at the mine site. The collapse inside the mine registered a 3.9 magnitude, they said.

Bodie Allred, a cousin of Kerry Allred, is normally a miner, but his job now is mine rescue supervisor.

"I know that the gentlemen in there know that we are on our way," Allred said Wednesday. "They know damn well that we're doing what we can to get to them, and we're going to do it. There is no doubt about it," he added.

Asked if he believed they were alive, Allred said, "Yes, I do. They're very tough men." E-mail to a friend